1. Technical Field
The present invention relates to a satellite orbit data compressing method, a satellite orbit data providing method, a satellite orbit data decompressing method, and a data compressing device.
2. Related Art
A global positioning system (GPS) is widely known as a position calculating system employing positioning signals and is used in position calculating devices built into mobile phones and car navigation apparatuses. In the GPS, a position calculating operation for calculating a three-dimensional coordinate value indicating the position of a target apparatus and a clock error is carried out on the basis of information such as positions of plural GPS satellites or the quasi-distance from the GPS satellites to the target apparatus.
In calculating a position using the GPS, satellite information such as the position, velocity, and moving direction of the GPS satellite is first calculated on the basis of navigation data such as almanac or ephemeris which overlaps with a GPS satellite signal emitted from the GPS satellite. The almanac serves as an influential key to capture a satellite but is not generally used in the position calculating operation, because it is poor in accuracy concerning the satellite orbit. On the other hand, the ephemeris serves as an influential key to capture a satellite and can also be used in the position calculating operation, because it is excellent in accuracy concerning the satellite orbit. Therefore, for example, when the position calculating operation is started without the ephemeris, the ephemeris should be acquired from the GPS satellite signal, thereby enhancing the TTFF (Time To First Fix).
Therefore, in a server and client system, a technique for allowing a server to predict long-term predicted ephemeris (long-term predicted orbit data), which is a kind of satellite orbit data and is the ephemeris corresponding to a long term of one week, and to provide the long-term predicted ephemeris to a position calculating device as a client was developed and disclosed, for example, in US-A-2002-188403 and US-A-2005-212700.
Since the long-term predicted ephemeris is data acquired by predicting satellite orbits of all the GPS satellites for a long term (for example, one week), the size thereof is much greater than that of the ephemeris for one satellite. Since the long-term predicted ephemeris is downloaded from a server system and used by a position calculating device, there is a need for reducing the size of the long-term predicted ephemeris so as to be as small as possible, thereby reducing communication cost of communication time and communication traffic.